How can one tell her fiancé about vitiligo, which was concealed from him after the marriage contract, and is concealing the disease considered a sin?

The majority of scholars hold that the defects necessitating disclosure before marriage, which establish the right to annulment, are those that impede sexual intercourse or are repulsive and contagious diseases such as leprosy and elephantiasis. Vitiligo is not included among these defects, so it does not establish the option of annulment, as it does not cause aversion. As long as this disease does not establish the option of annulment, it is not forbidden to conceal it from the suitor unless he stipulates freedom from it, because marriage is built upon generosity and mutual respect.